compat: add linux/of.h just when CONFIG_OF is set.
In kernel < 2.6.34 linux/of.h unconditionally includes asm/prom.h which
is not available on all architectures. In newer kernel versions this is
just included if CONFIG_OF is set. For these old kernel just include
linux/of.h when CONFIG_OF is set. This fixes a compile problem with
ath6kl as it uses linux/of.h only when CONFIG_OF is set.